The PI seeks to use the FIV/cat model to ascertain the role of the thymus as a reservoir for lentivirus replication and as a site for centralized T-cell destruction in pediatric lentivirus infections. His preliminary data indicate that mid-gestational exposure to FIV leads to widespread thymic replication of FIV, profound thymocyte depletion by the time of birth and to atypical regeneration by 3 months of age. He now proposes to directly inoculate individual kittens in mid-gestation (D.35), or day old neonates with virulent FIV-NCSU1. Kittens will be sacrificed at 25 and 90 days after FIV exposure. The effect of FIV replication on the thymus will be evaluated by simultaneous comparison of the extent of FIV replication (in situ hybridization and immunohistochemistry), changes in thymocyte phenotype, histomorphology and percent apoptotic cells (flow cytometry). The ability of stromal cells to support proliferation in co-stimulation assays of the regeneration thymus from FIV-infected kittens will be compared to the proliferation support provided by the thymus of 6 month old kittens sham-exposed in utero. Cytokines and hormones that influence thymus function will be evaluated using in situ hybridization of IL-1, IL-6 and GM-CSF and assessment of serum concentrations of cortisol, prolactin, and ILGF-1. The potential for thymus regeneration to enhance productive infection and to support centralized T-lymphocyte depletion will be determined after the pharmacologic induction of thymus atrophy in conjunction with neonatal FIV infection.